In optical networks, individual carrier signals (channels) are often multiplexed together to form a signal to be sent along optical links of the network. An example of such a technique is Wavelength Division Multiplexing (WDM), wherein different wavelengths of the multiplexed signal are used for each channel.
Networks comprise nodes (routers) for switching signals between optical links of the network. Many networks (referred to herein as multilayer optical networks) comprise switches (either nodes or parts of nodes) that operate on the multiplexed signal at different levels of granularity, with some switches able to divide the multiplexed signal into smaller segments (i.e. segments comprising fewer channels) than other switches of the network. For example, in a Synchronous Digital Hierarchy (SDH) network, the network is typically split into two layers, a high order layer comprising switches that operate at a granularity that is greater than one channel and a low order layer comprising switches that can operate on individual channels of the multiplexed signal. The high order layer is usually superior in cost effectiveness and scalability but the low order layer is more flexible in the allocation and grouping together of channels.
In such networks, routing algorithms are used to determine how to allocate network resources to satisfy traffic demands, the aim being to optimise the network resources such that as many demands as possible can be allocated. Accordingly, the routing algorithm for determining allocation of resources in the multilayer optical network will effect how efficiently the network operates. If the high order layer of the network is used too much, the bandwidth will be exhausted too fast as signals will utilise more bandwidth than would be the case if the signal was groomed of appropriate channels at more regular intervals. On the other hand, if the lower order layer is used too much, the grooming capacity of the lower order layer soon becomes unavailable.